Final answer:
The inability to begin M phase would not explain the cessation of proliferation of a mutant yeast strain when shifted to a higher temperature because cells would accumulate DNA instead of halting proliferation as observed.
Step-by-step explanation:
A student has asked why a mutant yeast strain stops proliferating when shifted from 25°C to 37°C. One possible explanation that would not explain the results with the mutant is the inability to begin M phase. If the cells are unable to begin M phase, this would result in a cell population that accumulates DNA, as the cells would prepare for division but not actually divide. This does not match the observed phenotype of cells that stop proliferating at the higher temperature.
In the context given, possible explanations such as the inability to initiate DNA replication, the inability to activate proteins needed to enter S phase, or the inappropriate production of a signal that causes the cells to remain in G1 are more likely to result in a stalled proliferation at the higher temperature because they all directly interfere with the progression of the cell cycle prior to the DNA replication event in S phase.